In addition to nitrogen, phosphor and potassium as main fertilizer components, the use of sulfur is increasing in importance in agriculture. Based on the environmental law, sulfur emissions from burning fossil materials have been drastically reduced in the last decade, which in the past represented an essential part of soil supply with sulfur. Moreover, besides ammonium sulfate, in the meantime almost no cheaper fertilizers have been available at the market. However, the use of it alone causes also a considerably bad nitrogen/sulfur relation which is compensated only because of its low price. Furthermore, a large part of ammonium sulfate is used in the form of fine crystals as wastes from the caprolactam industry, and it leads to bad storing and delivery properties of the related fertilizers.
Based on the fact that the production rate of ammonium sulfate from caprolactam processes continuously becomes less by process development and environmental loads due to sulfur emissions are to be kept absolutely small, an obvious demand for the cost-efficiently producible fertilizer granules having the optimal nitrogen/sulfur relation as well as good storing and delivery properties is recognizable on the market. The combination of urea with ammonium sulfate offers quite good prerequisites so that work has been done since several years to develop such a fertilizer.
The production of granules containing urea and ammonium sulfate is not a problem because of the properties of ammonium sulfate or urea/ammonium sulfate mixture. Ammonium sulfate melts at over 500.degree. C. so that the granulation of a mixed molten urea solution having a high ammonium sulfate content is excluded. Ammonium sulfate is soluble in a molten urea solution at usual molten urea temperature only up to about 20 wt. % and it can be therefore granulated homogeneously. If a higher ammonium sulfate content is required, so only compaction, granulation from solutions or granulation of ammonium sulfate/urea slurries are taken into consideration. Since the compaction process is excluded from the bulk market and the granulation process from solutions is unacceptably expensive and at the same time the ammonium sulfate content of more than 20 wt. % is desirable because of good application properties, the former proposals for solution have been based on the granulation of proper slurries. In this connection, an essential problem is that in case of an ammonium sulfate content of more than 60 wt. %, the granulation temperature of those slurries rises very quickly, and this shall be avoided because of the extremely strong formation of vegetable poison, biuret, from urea.
From the U.S. Pat. No. 3,785,796 a process to produce ammonium sulfate urea granules is known with which it is generally possible to produce granules having an ammonium sulfate content of 30 to 70 wt. %. In this case, commercial ammonium sulfate not milled is mixed with a molten urea solution and granulated in an "oil" or "dish" granulator, while the related slurry is dropped down. Both granulator types have the merit that they work in a quasi-dustfree manner so that a dust return can be deleted.
Disadvantageous to this process is the fact that granules have low quality due to their inhomogeneous structure caused by the granulation process. Furthermore, the "dish" and "oil" granulators have not been used in fertilizer granulation processes because their operating costs are relatively large, and it is difficult to achieve large production units by these processes. Finally it is also disadvantageous that the range of "granulation capability" of urea ammonium sulfate slurries mentioned in the U.S. Pat. No. 3,785,796 is adequate only for coarse-crystalline ammonium sulfate and cannot be technically realized with ammonium sulfate of smaller particle size.
According to EP-A 0 289 074, a process to produce urea ammonium sulfate granules of an ammonium sulfate concentration of 15 to 50 wt. % is described by which the ammonium sulfate granules introduced into the fluidized bed are sprayed by a liquid which contains 70 to 99.9 wt. % urea.
Demerits of this process are the inhomogeneity of ammonium sulfate and urea, necessity of using expensive ammonium sulfate coarse-crystals hardly available on the market and the impossibility to produce a granule diameter larger than 3 mm, as it is preferred by modern agriculture. The presented alternatives to the use of compacted ammonium sulfate of a larger basic diameter require a second forming stage which leads to extremely high operating costs.
According to U.S. Pat. No. 4,943,308, a process to produce ammonium sulfate urea granules of an ammonium sulfate content of 15 to 30 wt. % in the fluidized bed is disclosed. In this case, ammonium sulfate is dissolved in a 75 to 85 wt. % urea solution, concentrated to 92 to 97 wt. % and atomized by means of air after addition of granulation additives. In this process the maximum ammonium sulfate content of granules is limited by the maximum solubility of ammonium sulfate in the urea solution. Claimed is therefore only a maximum ammonium sulfate concentration of 30 wt. %.
Furthermore, as for this process it is also disadvantageous that in spite of the addition of crystallization aids which should reduce dust formation, considerable emission arises. Also according to the comparison experiment mentioned in this patent in which ammonium sulfate is mixed with a 95 wt. % urea solution by adding granulation aids and the ammonium sulfate is milled before granulation, the 30 wt. % ammonium sulfate content is not exceeded. Finally in this process two-material nozzles are used, while the atomization of slurries is done by means of highly compacted air. Since the mists generated so have a preferable bulk-specific diameter of 30 to 60 .mu.m, ammonium sulfate granules must be milled quite finely so that they are wetted sufficiently with liquid and solid ammonium sulfate granules are not separated from mists because of density difference to the urea solution.
Moreover, the described process functions only because a large part of ammonium sulfate is dissolved in the 95 wt. % urea solution because of a low ammonium sulfate concentration. In spite of the relatively high portion of dissolved ammonium sulfate in the slurry, a large quantity of ammonium sulfate (about 17 wt. %) is lost as emissions. If these emissions are recycled to the slurry surge tank, the viscosity of the suspension increases to such an extent that in case of an ammonium sulfate content &gt;30 wt. %, the process is interrupted due to plugging.